Chromates are powerful inhibitors of anodic and cathodic components of corrosion reactions. However, as chromates are dangerous pollutants and toxins, there is a great desire to eliminate their use in industrial surface finishing processes such as surface conversion. To successfully replace chromated surface finishing processes with functional attributes it is essential to understand how chromate works. The essential attributes of Cr chemistry leading to corrosion protection are summarized as follows.
On metal surfaces, particularly aluminum alloys, chromates are readily adsorbed and reduced to hydroxylated Cr3+. This surface complex appears to be exceptionally inert and strongly inhibits electron transfer reactions including oxygen reduction and further chromate reduction. The ability to inhibit oxygen reduction is a main component of corrosion protection afforded by chromate. Sub-part per million concentrations of chromate have been observed to reduce the oxygen reduction reaction rate to low levels. This potent inhibition process is made even more powerful because the adsorption and reduction reaction occurs on many different metals. This behavior likely accounts for the remarkable effectiveness of chromate passivation on various different metals and on microstructurally complex Al alloys.
Chromates also inhibit anodic reactions. Normally, resistance to pitting is only detected in environments where the chromate-to-chloride ratio exceeds 0.1. On this basis it might be argued that anodic inhibition is not as potent as cathodic inhibition. Nonetheless, it is believed to be important overall component of chromate corrosion protection.
Chromate conversion coatings (CCCS) provide protection to underlying substrates and intercoat adhesion in coating systems. Their most intriguing attribute is their ability to store and release a chromate corrosion inhibitor. While this attribute may lead to strongly inhibiting coatings, it is a temporary effect that is lost as the coating dehydrates under the influence of heat or dry environments. Long-term retention of self-healing characteristics represents an opportunity to improve Cr-free coating system performance.
Chromates are “suicidal inhibitors” in the sense that as they react with a metal surface, they stifle further electrochemical reactions; including the one that leads to the continued formation of the inhibiting film itself. For this reason, chromates by themselves do not lead to the formation of robust conversion coatings. To form CCCs, supplemental ingredients must be added to an aqueous solution to make it a coating bath. Supplemental ingredients include activators like fluorides, and accelerators like ferricyanide. In Al alloys, fluoride activates the surface by initially dissolving the protective oxide. This allows chromate reduction to proceed long enough for a three-dimensional film to form. Ferricyanide acts as a redox mediator and accelerates the rate at which the chromate reduction-aluminum oxidation redox couple proceeds. Once Cr3+ is formed near the Al surface, it hydrolyzes, polymerizes and condenses according to a sol-gel mechanism. This forms a Cr(OH)3 “backbone” consisting of linked octahedral units of hydroxylated Cr3+, which comprise the CCC film. As this backbone forms, chromates are adsorbed onto it. Chromate adsorption onto the backbone is reversible for a time, which leads to the famous self-healing effect when the CCC is contacted by an attacking electrolyte. In self-healing, chromates stored at adsorbed sites on the backbone are released into solution where they may be transported to defect sites to stifle further corrosion by the mechanisms discussed earlier. In this way, CCCs are able to store and release a potent corrosion inhibitor for self-protection.
CCCs are hydrated gels whose properties change as water is lost. Once removed from solution CCCs dehydrate. As water is lost, the backbone consolidates leading to shrinkage-cracking, immobilization of chromates, and loss of the self-healing characteristic and overall corrosion resistance. This process occurs over a matter of days in ambient indoor environments, and is dramatically accelerated by exposure to elevated temperatures or low humidity.
For aluminum alloys, it should be noted that chromate conversion coatings are often considered as a single process suitable for all alloys under all processing conditions. In reality, this is not the case since different formulations are used for different applications. Indeed there is no single, published database comparing the performance of chromate conversion coatings on a range of alloys cast or wrought in a range of tempers. The available performance data places a strong emphasis on sheet 2024-T3 with some data reported for 7075-T6 and 6061-T6 substrates.
It should also be noted that the conversion coating is a multi-step process usually involving both cleaning and deoxidizing/desmutting prior to conversion coating. Over many years, the metal finishing industry has optimized the pre-treatment steps for chromate conversion coating and it is not surprising that a chromate-based deoxidizer is often used since it sets up a surface more amenable to chromate conversion coating than other deoxidizers. Chromate alternatives may have their own requirements for pre-treatment, which may not be the same as the current process steps. These two factors should be taken into account when considering the use of chromate conversion coating replacements.
It is therefore a goal of the present invention to provide a chromate-free coating having the same ease of applicability and similar performance characteristics as chromate conversion coatings including the ability to self-heal. Furthermore, it is a goal of the present invention to provide a chromate-free coating process that can be carried out within the established pre-treatment procedures used in industry.
The prospect of replacing chromate conversion coatings has brought with it considerable investigation of potential alternatives based on a broad range of chemistries. Furthermore within each chemical category there is the potential for a broad range of formulations most of which will not yield a viable industrial process due to processing or performance limitations. Several reviews of the subject exist. These reviews show that a very broad range of approaches and chemistries has been considered. Several commercial Cr-free conversion coating technologies, and a somewhat greater number of primer coating technologies are available. n terms of chemistry, the large number of reports and patents related to Ce indicate that it is an excellent inhibitor of metal corrosion. Among non-Cr corrosion inhibitors, the mechanistic understanding of Ce inhibition is clearly the most developed. Other notable transition metal inhibitors are Mn, Co, V, W, Mo, and Fe. These are distinguished by the fact that they can strongly inhibit corrosion under the proper conditions and have been cited in many Cr-free coating patents. Sufficient intercoat adhesion is essential for durable coating systems. In recent years, silane coupling agents, and functionally graded or tailored sol-gel coatings have been explored for these purposes with some measure of success. These systems derive high adhesion from covalent bonding with the metal substrate and organic topcoats.
A comprehensive review of all CCC alternatives is difficult due to the range and quality of performance data for these processes, and because different processes are targeted towards different segments of the metal finishing industry that each have different performance requirements. Some comparative studies have been carried out and are a good source of performance data, but they do not include all the processes described herein. Furthermore, developments in chromate alternatives are progressing rapidly and results presented in comparative reports may not reflect the current performance of processes.
Chromate conversion coatings are used in a broad range of applications in industry, especially in aluminum finishing. An equally broad range of alternatives has been explored to meet the performance and processing requirements of different sectors of industry (Table 1). Currently, several chromate-alternatives have gained acceptance in specific sectors of the market. These markets can be divided into those that require protection in an unpainted state and those that require performance under paint. For the latter category, many alternatives demonstrate good performance characteristics. The aerospace industry falls into the former case and a drop-in replacement still does not exist in this high performance end of the market, which has very high standards for corrosion resistance of the unpainted conversion coated surface in the neutral salt spray test.
TABLE 1Major Classes of Chromate AlternativesCoating TypeIndustry SectorStatus1Titanium and ZirconiumSheet stock for canning,MatureFluorocomplexesAutomotiveDevelopingCerium-basedArchitecturalDevelopingAerospaceEvaluationCo-basedMarineDevelopingAutoDevelopingAerospaceEvaluationMo-basedSn and Galvanized ProductDevelopingHydrotalcitesAerospaceEvaluationMn-basedSome Sheet Product,DevelopingAerospaceEvaluationBoehmite CoatingsAerospaceEvaluationSilane CoatingsAutoDevelopingConducting PolymersFerrous MetalsEvaluationSelf Assembled MonolayersAuto Al/Mg AlloysDeveloping1Mature: in the industry for a number of years; Developing: may be introduced soon; Evaluation: still undergoing trials.
Table 1 lists the major types of chromate alternatives in use or under development and the industries that are currently targeted by the manufacturers of these products. The majority of these processes are still under development with fluorozirconoic and fluorotitanic acid coatings being the most mature of the replacement technologies, with products in the market for a number of years.
The present invention provides a general approach for the formation of a corrosion resistant coating with self-healing characteristics based on contacting metal surfaces with aqueous solutions whose primary film-forming agent is vanadate.